The invention relates to a cathode ray tube comprising an electron gun having a beam-shaping portion having at least one electron source for generating an electron beam, a prefocusing system of electrodes across which a prefocusing voltage is supplied during operation so as to form an electron-optical prefocusing lens, the electron gun further comprising a main lens system of electrodes whose outer electrodes are provided with means for supplying, at least during operation, a main focusing voltage between said electrodes so as to form an electron-optical main focusing lens, said electrodes being provided with at least one aperture for passing the electron beam and with securing means with which the electrodes are secured to an insulating supporting body.
The invention also relates to an electrostatic lens system comprising a prefocusing system of electrodes across which a prefocusing voltage is supplied during operation so as to form an electron optical prefocusing lens, a main lens system of electrodes whose outer electrodes are provided with means for supplying, at least during operation, a main focusing lens, said electrodes being provided with at least one aperture for passing an electron beam and with securing means with which the electrodes are secured to an insulating supporting body.
A device of this type is known from U.S. Pat. No. 4,168,452. The electron gun used in this device is of the tripotential type, in which the outer electrodes of the main lens system are provided with means in the form of electric connections for supplying therebetween, at least during operation, a comparatively large main focusing voltage of the order of 15-20 kV. As in other types of guns, in which a comparatively large main focusing voltage is supplied between the outer electrodes of the main lens system, as in, for example most conventional bipotential guns, this generally has the consequence that during operation only a moderate potential is present on the first electrode of the main lens system, in contrast to unipotential guns in which the outer electrodes of the main lens system both convey the same high potential. Since the first electrode of the main lens system generally also constitutes the last electrode of the prefocusing system, this leads to a moderate prefocusing action of the gun and hence to a poor. beam definition in a device of the type described in the opening paragraph.
To avoid the latter phenomenon, an additional electrode is used in the prefocusing system in the electron gun of the known device, which electrode conveys a higher potential during operation than the first electrode of the main lens system. Thus, a higher field strength in the prefocusing system is realised so that a better prefocusing is obtained, which leads to a sharper electron beam.
A drawback of the known device is, however, that an extra electrode is required for the realised improvement of the beam definition, which does not only have a cost-increasing effect but also requires more space and, under circumstances, an extra electrical connection in the gun.